Digital CD (compact disk) format optical systems were originally developed for mass distribution of music (audio files) with higher quality reproduction than was possible with analog tape or record systems. CD-ROM systems are an offspring of audio CD systems and used for mass distribution of computer-readable information (which can include audio, video and text as well as conventional computer "data"). Both audio CD's and CD-ROM's are pre-recorded by a stamping process and cannot be recorded onto by the user. New systems are being marketed to permit the user to record information on the media once (CD-R); newer systems that permit the user to record and erase information and to record new information onto the media (CD-E).
The format in which data is recorded onto CD-R and CD-E optical media (collectively referred to herein as CD-R/E) is substantially the same. Under current industry standards, each disk can hold up to 650 MB of data partitioned into up to 99 tracks (equivalent to 99 independently addressable files) when recorded during a single session. This limit is imposed by the number of entries that can be made in the Table of Contents (TOC) recorded on the disk. (It should be noted that efforts are being made to increase the disk capacity). A disk may also be partitioned into up to 45 sessions (and each session may be partitioned into one or more tracks) limited by the capacity of the disk to contain the recorded data and the required inter-session gaps. Consecutive sessions are separated by an inter-session gap (11,400 physical blocks for the first gap and 6,900 physical blocks for subsequent gaps) which includes a lead-in area (LIA) and a lead-out area (LOA). When recording, the optical head of the CD-R/E drive is located at the beginning of the recordable area of the disk (at the end of the last session if a multi-session disk), the inter-session gap is allocated, and one or more tracks of data is recorded. At the completion of recording the session, the LIA and LOA are written and the TOC (located in the program memory area (PMA) of the disk prior to the first user recordable area) is updated in a process known as "fixing". Recorded as a part of the LIA and the LOA is a copy of the TOC containing information regarding the starting track address of the current session as well as of all prior recorded sessions and the starting address of each track. When all of the data for the session has been recorded, the LIA and LOA recorded, the TOC is updated and the session is deemed to be "fixed"; the just recorded information cannot be re-recorded, corrected or otherwise modified in the same session area. Any modified data must be recorded in a new session and the TOC updated. Incremental packet recording is similar but with the input data broken into fixed length (or, in some versions, variable length) "packets", each with a link block, a LIA, a LOA and the data field. There is no limit on the number of packets which can be recorded, up to the available capacity of the disk.
There are currently no reliable methods for detecting and correcting errors encountered during the recording process of CD-R or CD-E media. These recording errors resulting from physical defects in the media, surface scratches, finger prints and other contamination can all cause permanent errors on read back rendering all or part of the data unrecoverable or defective. While a one or two consecutive bit unrecoverable error might not be a significant problem if audio or video information is involved, such an unrecoverable error in computer data is very serious.
In contrast, when data recorded onto magneto-optical (MO) or write-once, read-many (WORM) optical media is found to be defective, the data contents of the defective sector can be re-written to a spare sector in a designated spare area on the disk and a defect management table updated with a pointer from the defective sector to the replacement sector. The defect management table is then used to re-direct all references to the defective sector to the replacement sector, thus avoiding an error on read back.